Oil country tubular goods such as tubing and casing used in the excavation of oil wells and gas wells have a length in the range of about 10 to 20 meters. At an excavation site, they are connected end to end by means of threaded joints until the connected length is sufficient to reach oil or gas (typically at least 2,000 meters). In recent years, in order to cope with an increased demand for energy, deeper oil wells are being developed, and nowadays oil wells having a depth of from 8,000 meters to 10,000 meters are not unusual. In addition, tubing through which fluid such as a crude oil runs is surrounded by a plurality of casing pipes having different diameters from each other. Therefore, the number of oil country tubular goods which are connected to each other during excavation of an oil well amounts to a huge number. Thus, threaded joints which connect an extremely large number of oil country tubular goods are exposed to a very severe environment of use, since they are subjected to an axial tensile force caused by the weight of the oil country tubular goods and couplings, if couplings are used for connection, combined internal and external pressures, and geothermal heat. Accordingly, threaded joints are required to have high performance capable of maintaining gastightness without undergoing damage even in such a severe environment.
During the process of lowering tubing or casing into an oil well, due to various problems, it is sometimes necessary to lift a threaded joint which has been once connected out of the oil well, disconnect it, reconnect it, and then relower it into the well. API (American Petroleum Institute) requires that so-called galling (unrecoverable severe seizing) not occur even if makeup (connection) and breakout (disconnection) are repeated ten times for a joint for tubing or three times for a joint for casing, which has a larger diameter than tubing and is more susceptible to galling.
A typical threaded joint for pipes used for connecting oil country tubular goods to each other has a pin-box structure. A pin is a joint component having male threads typically formed on the outer surface at each end of an oil country tubular good, and a box is a joint component having female threads typically formed on the inner surface of a coupling (a threaded connector).
In a threaded joint referred to as a premium joint which has superior gastightness, unthreaded metal contact portions are formed at the tip of the male threads of the pin and at the base portion of the female threads of the box. The unthreaded metal contact portions can include a metal seal portion formed on a cylindrical surface of the pin or the box, and a torque shoulder which is nearly perpendicular to the axial direction of the threaded joint.
When oil country tubular goods are connected to each other by such a premium joint, one end of an oil country tubular good, which constitutes a pin, is inserted into a coupling, which constitutes a box, and the male threads of the pin and the female threads of the box are tightened until the torque shoulder portions of the pin and the box are brought into contact with each other with a prescribed amount of interference. As a result, the metal seal portions of the pin and the box establish tight contact with each other with a certain interference to form a metal seal due to direct metal-to-metal contact and achieve gastightness.
The surfaces of the pin and the box which contact each other when a threaded joint is connected are hereunder referred to as contact surfaces. The contact surfaces include the threaded portions of the pin and the box (the portions having male threads and female threads, respectively) and the unthreaded metal contact portions thereof (namely, the metal seal portions and the torque shoulders of the pin and the box).
During makeup of a threaded joint, an extremely high pressure which may exceed the yield point of the material which constitutes the threaded joint is applied to the metal seal portions and the torque shoulders, namely, the unthreaded metal contact portions of the joint. Therefore, galling easily occurs particularly in the unthreaded metal contact portions of a threaded joint. In order to improve galling resistance and gastightness, a lubricant, particularly a viscous liquid grease (dope) called compound grease is generally applied to the threaded portions and the unthreaded metal contact portions, namely, the contact surfaces of a threaded joint before makeup of the joint. Compound grease also provides anticorrosive properties to the contact surfaces. With the intention of improving the retention of compound grease and the sliding properties achieved thereby, it is known to roughen the contact surfaces of a threaded joint by appropriate surface treatment (such as phosphate chemical conversion treatment or plating).
Compound grease contains large amounts of powders of relatively soft heavy metals such as zinc, lead, and copper in order to achieve the desired lubricating and anticorrosive properties. However, grease which has been applied to the contact surfaces of a threaded joint is squeezed out onto the exterior of the joint during makeup or washed off when the joint is recoated with compound grease prior to reconnection, and there is the possibility of its being discharged into soil or into the ocean, thereby producing harmful effects on the environment and especially on sea life. In addition, since application of compound grease to a threaded joint is carried out in the field whenever the joint is made up, use of compound grease worsens not only the working efficiency of makeup but also the working environment particularly due to the harmful effect of lead on humans. Therefore, there is need for development of a threaded joint capable of being made up without application of compound grease.
A threaded joint which can be used without application of compound grease and which has a solid lubricating coating formed on the contact surfaces was known in the art. For example, JP 09-72467 A1 (Patent Document 1) discloses a threaded joint having a lubricating resin coating in which molybdenum disulfide (MoS2) or tungsten disulfide (WS2) is dispersed.
Such a threaded joint having a solid lubricating coating greatly reduces harmful effects on the environment and humans compared to compound grease. Since a threaded joint is shipped after the solid lubricating coating is formed, application of a lubricating grease before makeup operations in the field can be eliminated, leading to an improvement in the working efficiency and working environment.
However, a solid lubricating coating of the type described above has poor ductility and fluidity, and it tends to easily peel off. Therefore, if an excessively high pressure is locally applied to a part of the threaded portions or unthreaded metal contact portions of a threaded joint during makeup to such an extent that it causes local plastic deformation, the solid lubricating coating in that part of the threaded joint peels off so that the naked metal surface is exposed. Even if the exposed area is small; it may instantly cause galling.
In contrast, when a liquid lubricant such as a lubricating grease including compound grease or lubricating oil which is liquid and fluid at room temperature is applied to the contact surfaces of a threaded joint, the lubricant retained in the gaps between male and female threads or in recesses due to surface roughness can seep out under the pressure exerted during makeup, so even if an excessive pressure is locally applied to a part of the contact surfaces of a threaded joint, the lubricant can move to that part and thereby prevent galling. This action is called the self-repairing ability of liquid lubrication. In general, the higher the fluidity (or the lower the viscosity) of a liquid lubricant, the higher is its self-repairing ability. Accordingly, as long as galling resistance is concerned, a liquid lubricating coating having fluidity is generally advantageous.
However, if a liquid lubricant is applied to the contact surfaces such as the threaded portions and unthreaded metal contact portions of a threaded joint at the time of shipping, the liquid lubricant makes the surfaces to which it is applied sticky, and foreign matter such as dust, sand, or debris easily tends to adhere to the contact surfaces. In particular, when oil country tubular goods are stood upright during assembly in the field, rust flakes and blasted grains fall along the walls of the tubular goods. If the contact surfaces are sticky, a large amount of such foreign matter adheres to the surfaces. As a result, even if a lubricating grease which is expected to exhibit a self-repairing ability is applied, its lubricity is significantly deteriorated, and galling may easily occur when makeup and breakout are repeated. Namely, from the standpoint of adhesion of foreign matter, a solid lubricating coating having a dry surface is advantageous.
US 2004/0239105 A1 (Patent Document 1) discloses a threaded joint having contact surfaces coated with a lower liquid grease layer and an upper solid lubricating layer. With this threaded joint, the above-mentioned problems of a solid lubricating coating and a liquid grease are both alleviated so as to attain the advantages of these two types of lubricating coatings at the same time. However, due to the presence of a liquid grease as a base layer, the amount of reduction in surface stickiness is inadequate. In addition, due to the very soft nature of the layered coating, if the coating is struck by an object, it is readily deformed or it is removed and adheres to the object, thereby reducing the effect of the coating.
JP 11-63132 A1 (Patent Document 2) and JP 11-223260 A1 (Patent Document 3) disclose that a member made of a lubricating oil-containing polymer (also called an oil-impregnated polymer) is disposed in the vicinity of a ball screw or bearing which needs lubrication such that lubricating oil is continuously supplied by seepage from that member.
A lubricating oil-containing polymer is a solid material consisting of a lubricating oil and a thermoplastic organic polymer which are mutually dissolved so as to form a single phase. A polyolefin resin in particular can contain a large amount of a lubricating oil. In particular, hers a combination of a polyethylene and mineral oil can contain as much as 70% of lubricating oil (mineral oil). A lubricating oil contained or impregnated in a polymer can exert its lubricating effect by moving from the inside toward the surface to seep out under the action of factors such as pressure, temperature rise, and the like. Such a lubricating oil-containing polymer can be manufactured by heating to melt a mixture of starting substances which are an organic polymer and a lubricating oil, casting the resulting melt in a mold, and then cooling the mixture in the mold under pressure until the melt solidifies.    Patent Document 1: US 2004/0239105 A1    Patent Document 2: JP 11-63132 A1    Patent Document 3: JP 11-223260 A1